In a waste treatment process, contaminants need to be removed before disposal. Insoluble contaminants can be physically separated and disposed of and soluble contaminants can be biologically metabolized. Bacteria may be used to break down or metabolize the soluble contaminants. The bacteria are often part of a composition called activated sludge. Once the bacteria have been used to break down the soluble contaminants, a portion of the activated sludge, referred to as return activated sludge, is recycled back into the process to continue metabolizing contaminants in the waste stream. The rest of the waste activated sludge is usually disposed of.
In another part of the waste treatment process, chemical coagulants may be used to aggregate insoluble and a small percentage of soluble contaminants (<15%) for separation and removal. Chemical coagulants are expensive.